1. Field of the Invention
The present invention relates to an electric car air-conditioning apparatus having a motor-driven compressor driven by the power supplied from a battery.
2. Description of the Related Art
The operation of a conventional electric car air-conditioning apparatus will be described with reference to a circuit diagram of a conventional electric car air-conditioning apparatus shown in FIG. 4.
When a battery 2 is connected, a capacitor 11 is charged with a time constant determined by a product of the resistance value of a resistor 8 and the electrostatic capacity value of the capacitor 11. If an input voltage detecting means 7 of a motor-driven compressor driving unit 1 detects a voltage larger than a predetermined value, a switching device 3 is closed by a control section 6 so that power is supplied from the battery 2 through the switching device 3, so that the air-conditioning motor-driven compressor 5 is driven by the motor-driven compressor driving unit 1.
The resistance value of the resistor 8 must be large enough to restrain a charging rush current in order to prevent a fuse 10 from breaking. On the other hand, the resistance value must be small enough so that the fuse 10 is broken upon occurrence of a failure such as damage of the motor-driven compressor driving unit 1 or short-circuiting of the capacitor 11 or the like. Since the fuse 10 must not be broken in a normal operation current, the value of the fuse 10 is set to about 10 A. Accordingly, in order to flow a current about 40 A to break the fuse 10 surely upon occurrence of a failure, the resistance value of the resistor 8 is set to 7.5 ohm when the voltage of the battery 2 is 300 V.
In the above-mentioned circuit, in the case where the fuse 10 does not break at the time of a half-failure of the motor-driven air compressor driving unit 1 or the capacitor 11 or the like, that is, for example, in the case where a current of 9 A flows, the resistor 8 consumes a large power of (9 A).sup.2 .times.7.5 ohm=607.5 W. Accordingly, overheating causes deterioration of reliability such as smoking, solder dissolving, wiring damage, and so on. It is therefore necessary to provide a radiator or the like as a countermeasure to overheating. Accordingly the air-conditioning apparatus becomes large in size.
When the input voltage to the motor-driven compressor driving unit 1 does not rise in the case where the capacitor 11 is connected with its polarity reversed, the input voltage detecting means 7 does not detect the fact that the input voltage has exceeded a predetermined value, so that the control section 6 does not close the switching device 3 and the resistor 8 continues to flow a current into the capacitor 11. On the other hand, even in the case where the capacitor 11 is connected with its polarity reversed directly to the battery 2 and the input voltage to the motor-driven compressor driving unit 1 rises, the input voltage detecting means 7 detects the fact that the input voltage has exceeded the predetermined value and the control section 6 closes the switching device 3. Accordingly, a current continues to flow. As a result, the capacitor 11 is broken, and matters blown out of the capacitor 11 pollutes the air-conditioning apparatus to lower the performance and reliability of the apparatus. The capacitor 11 is a large-size one which has a rated voltage of about 300 V and an electrostatic capacity of about 1000 .mu.F. Accordingly, the failure of the capacitor 11 has a large influence.
Further, if a failure occurs in the switching device 3 or a driving circuit of the switching device 3, the air-conditioning motor-driven compressor 5 is driven by the motor-driven compressor driving unit 1 with the switching device 3 kept not-closed. In this case, the current of the motor-driven compressor 5 flows into the resistor 8. Accordingly there occurs a situation similar to the above-mentioned situation where the fuse 10 does not break.